In a toroidal continuously variable transmission, input and output discs are disposed on a main shaft, and a power roller is disposed between and in frictional engagement with the input and output discs. The input disc is fixedly mounted through ball splines on the main shaft. Belleville springs are usually disposed at an opposite side of the input disc with respect to the output disc to apply a pre-load to the input disc in order that the input and output discs are in frictional contact with the power roller at a suitable pressure. It is usual that the axial contraction amount of the belleville springs are restricted by a loading nut which is screwed on the main shaft and fixed at a position near a step section of the main shaft. Such a toroidal continuously variable transmission is disclosed, for example, in Japanese Patent Provisional Publication No. 4-69439.
However, drawbacks have been encountered in the above conventional toroidal continuously variable transmission, as discussed below. That is, the toroidal continuously variable transmission of this type is arranged such that rotation of a cam flange connected to an input shaft is-transmitted through a loading cam mechanism to the input disc, and then the rotational power of the input disc is further transmitted through the power roller to the output disc. Particularly in case that the transmission is of the double cavity type, the loading nut screwed and fixed on the main shaft is higher in rigidity (or spring constant) and difficult to have a sufficient axial length of its internal threaded portion from the viewpoint of construction of the transmission. Additionally, it is also difficult that the length of an abutting end section (contactable with the main shaft step section) of the loading nut is sufficiently large to compensate the tightening force of the loading nut. Under such a situation, when relative rotation (in an amount corresponding to play of each ball spline in the rotational direction) is made between the input disc and the loading nut while the main shaft elongates owing to a thrust under a large input torque change, the tip end of the abutting end section of the loading nut separates from the step section and therefore there is the fear of the loading nut being loosened.
Additionally, during operation of the above conventional transmission, the belleville springs make their axial sliding movement and axial contraction on the cylindrical section of the loading nut, in which the cylindrical section is subjected to a large amount of wear owing to sliding contact between it and the belleville springs. In view of this, the peripheral surface portion of the cylindrical section of the loading nut has been usually hardened, for example, by carburizing. In making such a hardening processing to the loading nut, it is necessary to make a precise control in order to avoid a strength degradation (depending on hardening) of the internal thread portion of the loading nut, thereby complicating the production process of the loading nut.